Light-emitting diodes or “LEDs” include thin layers of semiconductor material of two opposite conductivity types, referred to as p-type and n-type. The layers are disposed in a stack, one above the other, with one or more layers of n-type material in one part of the stack and one or more layers of p-type material at the other end of the stack. For example, the various layers may be deposited in sequence on a substrate to form a wafer. The junction between the p-type and n-type material may include directly abutting p-type and n-type layers, or may include one or more intermediate layers which may be of any conductivity type or which may have no distinct conductivity type.
Electrodes are connected to the n-type and p-type layers near the top and bottom of the stack. The materials in the electrodes are selected to provide low-resistance interfaces with the semiconductor materials. The electrodes, in turn, are provided with pads suitable for connection to wires or other conductors which carry current from external sources. The pad associated with each electrode may be a part of the electrode, having the same composition and thickness of the electrode, or may be a distinct structure which differs in thickness, composition, or both from the electrode itself. The term “electrode-pad unit” is used in this disclosure to refer to the electrode and pad, regardless of whether the pad is a separate structure or merely a region of the electrode. The wafer is cut apart to form individual dies which constitute separate LEDs.
In operation, electric current passing through the diode is carried principally by electrons in the n-type layers and by electron vacancies or “holes” in the p-type layers. The electrons and holes move in opposite directions toward the junction, and recombine with one another at the junction.
Energy released by electron-hole recombination is emitted as light. As used in this disclosure, the term “light” radiation includes infrared and ultraviolet wavelength range, as well as the visible range. The wavelength of the light depends on factors including the composition of the semiconductor materials and the structure of the junction.
LEDs formed from certain semiconductor materials normally use nonconductive substrates to promote proper formation of the semiconductor layers. The nonconductive substrate typically is left in place. For example, gallium nitride-based materials such as GaN, AlGaN, InGaN and AlInGaN are used to form LEDs emitting light in various wavelength ranges including blue and ultraviolet. These materials typically are grown on insulating substrates such as sapphire or alumina.
LEDs incorporating an insulating substrate must include a bottom electrode at a location on the stack above the substrate but below the junction. Typically, the upper layer or layers of the stack are removed in a region of the stack, so as to provide an upwardly-facing lower electrode surface on a layer at or near the middle of the stack in each die. This leaves a region referred to as a “mesa” projecting upwardly from the lower electrode surface and covering the remaining area of the die. The area of the die occupied by the lower electrode surface does not emit light. It is desirable to keep the horizontal extent of this inactive area as small as possible. In other LEDs, the upwardly facing lower electrode surface is not formed and a lower electrode is formed on the bottom surface of the stack of semiconductor materials. In these devices, the insulating substrate at the bottom of the stack is omitted.
In either type of LED, the top electrode typically is formed on the top surface of the stack, i.e., the top surface of the top semiconductor layer. Typically, the layers in the stack above the junction are transparent, so that light emitted at the junction can pass out of the stack through the top surface. The top electrode is arranged so that it does not block all of the emitted light. For example, an opaque top electrode may cover only a small portion of the top surface of each die. However, “current crowding” or “current bunching”, results in light emission concentrated in that area of the junction beneath the electrode, precisely where it will be most effectively blocked by the electrode. The amount of useful light reaching the outside of the die per unit of electrical current passing through the die, commonly stated as the external quantum efficiency of the die, is reduced by this phenomenon. Current crowding can also occur in the lower region, so that light emission is concentrated in the area of the junction near the lower electrode. Current crowding is a significant consideration with LEDs formed from materials having relatively high electrical resistivity, such as the gallium nitride-based materials.
To alleviate the current crowding problem, LEDs have been provided with transparent top electrodes, formed from thin layers of metals and metal compounds. A pad, which is typically opaque, is connected to the transparent electrode and occupies a small portion of the top surface. The transparent top electrode spreads the current in horizontal directions from the pad, so that current flow down through the stack is spread more evenly over the horizontal extent of the mesa.
Some of the light generated by the LED is subject to total internal reflection and is not emitted by the LED. The principle of total internal reflection is shown in FIG. 1, which shows a light source 32 generating a number of distinct light rays. The light rays pass through a first transparent material 34 having an index of refraction n, that is greater than the index of refraction n2 of a second transparent material 36. A first light ray 40 that is emitted by light source 32 is normal to the interface 38 between the transparent materials and is not altered when passing into the second transparent material 36. However, the direction of travel of a second light ray 42 is altered as it passes from the first transparent layer 34 to the second transparent layer 36. The third light ray 44 is incident upon interface 38 at the critical angle θc, as determined using Snell's law, and does not pass into the second transparent layer 36 but is directed along interface 38. Any light ray having an incident angle that is greater than the critical angle θc, such as fourth ray 46, is subject to total internal reflection and will be reflected back into first transparent layer 34.
The effect of total internal reflection in an LED will be discussed in connection with the example in FIG. 2. A light-emitting diode (“LED”) comprises a semiconductor structure of semiconductor material of two opposite conductivity types, typically referred to as p-type layers 20 and n-type layers 22. The p-type layers 20 and n-type layers 22 are disposed in a stack, one above the other. The LED includes a junction 24 provided at the interface of the p-type and n-type layers. Typically, the stack comprises a wafer for forming a plurality of LED devices. The wafer is then cut apart to form individual devices which constitute the separate LEDs.
FIG. 3 shows a plan view of a semiconductor wafer 48 incorporating a stack of semiconductor materials for a plurality of LED devices 50. The semiconductor wafer 48 includes a plurality of lines printed onto the upper surface of the wafer, called streets 52 and rows 54. The streets 53 and rows 54 divide the devices 50 of the wafer 48. The figures are not to scale and the streets and rows are generally not visible to the human eye but are schematically shown in FIG. 3. The semiconductor wafer 48 is typically separated into the individual devices 50 along the streets 52 and rows 54 using the cutting surface of a cutting wheel, or a conventional laser to cut the upper surface of the wafer.
FIG. 4 shows a conventional method of separating individual devices from a wafer. In FIG. 4, the laser beam 56 is swept across the upper surface 60 of semiconductor wafer 48 to divide the wafer into individual devices. The laser beam is substantially rectangular and approximately 25–35 micrometers wide and 500 micrometers long. The rectangular laser beam image cuts away a portion of wafer 48 to form a substantially rectangular cut 62. The individual devices remain connected by a portion of the wafer underlying the cut. The portion of the wafer 48 underlying the rectangular cut 62 may be called the kerf 64.
FIG. 5 shows a cross-sectional view of an individual device severed from a semiconductor wafer using the rectangular laser beam shown in FIG. 4. The LED is encapsulated in a substantially transparent encapsulant material 130 to form an LED package 128. Encapsulant layer 130 has an index of refraction n2 that is less than the index of refraction n1 of transparent substrate 26. As used herein, the term “refraction” means the optical phenomenon whereby light entering a transparent medium has its direction of travel altered. As a result, when the incident angle θi of a light ray 146 at interface 164 is greater than the critical angle θc, the light ray 146 is totally internally reflected back into the substrate 26 and does not pass into the encapsulant layer 130 where the ray can be emitted from the LED package 128.
Because the index of refraction n2 of the substantially transparent substrate 26 is greater than the index of refraction n3 of the transparent encapsulant 130, many of the light rays generated by the LED will not be emitted from the LED package 128, but will be subject to total internal reflection. The optical phenomenon known as total internal reflection, causes light incident upon a medium having a lesser index of refraction (e.g. encapsulant layer) to bend away from the normal so that the exit angle is greater than the incident angle. The exit angle will then approach 90° for some critical incident angle θc, and for incident angles θi greater than critical angle θc there will be total internal reflection of the light ray. The critical angle can be calculated using Snell's Law.
In many optoelectronic device packages, the light rays generated are never emitted from the package because such light rays are totally internally reflected within the various layers of the package. Thus, there is a need for packages having designs that optimize the amount of light emitted therefrom.